Trained Eye


Bulging Plaster Walls


Good day Mr Marantz,

I regularly read your column for any tips you have on home ownership. We recently bought a house in River Heights that was built in 1944. It, of course, came with plaster walls. After moving in and starting to repaint I noticed some bulges in the plaster. It is most prevalent under the windows, which are original. When I push on these bulges it collapses and a portion of the wall falls off exposing a rough layer underneath. Could you tell me what would have caused this and how do I go about repairing it. Can I use drywall compound?

Thank you for any information you can provide.


As you have already alluded to, cracks and damage to older plaster walls is inevitable in older homes like yours. The older plaster will dry out and crack along studs, over doors and windows, and other areas due to shifting and settlement in the home. Bubbling and flaking is less common and is usually due to moisture issues in the wall cavity or on the wall surface, itself. Repairs to the older plaster surface may not differ in either situation, but the underlying cause of the moisture should be addressed before attempting repairs in the later case.

Moisture damage to old plaster is most commonly found under and around windows, as is the situation in your home. This is due to moisture in one of several causes, but all are related to the old windows. We will look at three possible causes and repairs to the moisture before we address the actual plaster repair method. The three most typical sources of the water in your home are leakage due to rot in the window or trim, excess condensation and dripping on the interior of the window, and finally air leakage around the older window frame and sill.

The first place too look is the exterior of the windows themselves. Take a scratch awl or flat-head screwdriver and gently probe the wood window, sill, and brick mould in places where the paint has peeled or where deterioration is noted. If the probe easily enters the wood, push a little harder and see if the wood is rotten deeper than just on the surface. If this happens, the only solution is replacement of the wood. If the rot is in the window sash, then it is definitely time to take out the credit card and buy a new window unit. If the rot is only in the brick mould or sill, these may be easily replaced and painted to match the existing window. Also check around the brick mould for gaps and caulk with a good quality exterior sealant if spaces are prevalent.

Excess condensation on the interior of the windows may only occur in the dead of winter, so this will be difficult to determine at this time of year. Some signs to look for now would be peeling of paint and moisture stains only near the bottom of the window sash, the sill, and below the window. The inside of the bottom window frame will also be stained and damaged when this occurs, as well. If this is the cause of problem, plaster repairs may be possible now, but air sealing the window and indoor air quality issues will also be required to be addressed. I am sure that will be discussed in a future column, and has been visited several times before.

The final potential cause of the moisture, and possibly the most common, is condensation within the wall cavity under the old window or around the frame. This will occur due to a lack of proper insulation and air sealing between the window frame and the wall studs and plaster. This may only be identified by prying off the inside window casing, chipping away some plaster next to the window and looking at the size of the gap in this area. If the gap is more than ¼ inch without insulation, then cold air infiltration and condensation is certain. This condensation will freeze in colder temperatures and thaw when it warms up, leading to leakage through the inside plaster. This typically causes the bubbling paint and plaster that you have described. To fix this, take off all the window casings around the windows in question, cut away the plaster covering the gap and fill the gap with low-expansion foam insulation. Once dry, this insulation can be cut flush with the plaster, the casing re-installed and caulked around the perimeter, and the problem may be solved.

Once the moisture issues are dealt with, patching the plaster may be relatively easy, or quite difficult, depending on how extensive the moisture damage is. If just the surface, white section of the plaster is damaged or gone, then patching with drywall compound will work fine. If the under mortar layer is damaged or flakes off easily and turns to sand, the repairs are more extensive. In that scenario, this will have to be patched with a stronger wall patch or drywall board before adding a top layer of drywall compound.

In either case, several layers of “mud” may have to be applied, allowing proper drying time between coats, before finish sanding and painting of the old exterior plaster wall.




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